Date: Sun, 10 Oct 2004 22:21:20 -0500 From: Dan Nelson <dnelson@allantgroup.com> To: "Li, Qing" <qing.li@bluecoat.com> Cc: freebsd-hackers@freebsd.org Subject: Re: Bit field definition ? Message-ID: <20041011032120.GA10108@dan.emsphone.com> In-Reply-To: <00CDF9AA240E204FA6E923BD35BC643606BF695A@bcs-mail.internal.cacheflow.com> References: <00CDF9AA240E204FA6E923BD35BC643606BF695A@bcs-mail.internal.cacheflow.com>
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In the last episode (Oct 10), Li, Qing said:
> > In the last episode (Oct 08), Li, Qing said:
> > > The bit fields "th_x2" and "th_off" in "struct tcphdr", even
> > > though defined as "u_int", actually occupies 1 byte.
> >
> > u_int th_x2:4, /* (unused) */
> > th_off:4; /* data offset */
> >
> > The :4 after each variable means 4 bits long, so both fields
> > together take up 8 bits = 1 byte. That's the whole purpose of
> > bitfields :)
>
> D'oh
>
> I didn't ask the right question.
>
> It seems u_int specifies the packing and alignment size
> for the bit fields, is that correct ?
I don't think so. C99 only allows bitfields to be of type Bool, signed
_int, or unsigned int, so that seems to prevent the use of
char/short/int/long to dictate padding or alignment. There must be
something in the FreeBSD ABI that says structs must be padded so they
are 4-byte aligned, even if none of the members require it. Try putting
your 4 structs into a program and compiling them with gcc -Wpadding:
> struct {
> u_int a:4,
> b:4;
> }; is 4 bytes in size.
a.c:7: warning: padding struct size to alignment boundary
> struct {
> u_int a:4,
> b:4;
> short c;
> }; is 4 bytes in size.
a.c:13: warning: padding struct to align 'c'
(1 byte of padding added just before c)
> struct {
> u_int a:4,
> b:4;
> short c;
> u_char d;
> }; is 8 bytes in size;
a.c:19: warning: padding struct to align 'c'
(1 byte padding just before c, and 3 bytes just after d). I think it
should have printed a "padding struct size to alignment boundary"
warning also, since if it didn't, the padding after d would have been 1
byte, and struct would have been 6 bytes total.
> But
>
> struct {
> u_int a:4,
> b:4;
> u_char d;
> short c;
> }; is 4 bytes in size;
>
a.c:21: warning: padding struct size to alignment boundary
This last warning I don't understand, since 1+1+2 is 4 all by itself.
No padding is needed or used.
--
Dan Nelson
dnelson@allantgroup.com
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